One of the most error prone components of a modern high voltage transformer is the transformer bushings. The transformer bushing relays the incoming high voltage to the windings of the transformer while providing electrical insulation. A common type of transformer bushing has a center conductor with wrapped layers of insulating oil impregnated paper arranged within a porcelain body. For testing and monitoring purposes the transformer bushing often comprises a bushing tap adaptor. The bushing tap adaptor is connected to one of the oil impregnated paper layers such that a fraction of the high voltage feeding the center conductor becomes accessible through the bushing tap adaptor. The bushing tap adaptor is configured to receive a bushing sensor.
The transformer bushing must withstand high electrical fields, large temperature differences, as well as high humidity.
These harsh conditions often cause transformer bushings to fail and problems related to transformer bushings are responsible for a significant portion of all transformer related errors. The transformer bushing errors that occur are usually related to the condition of the bushing insulation.
It is therefore important to monitor the condition of the insulator in the transformer bushing. A first method to assess the condition of the insulator involves estimating the loss tangent and capacitance by means of comparing the voltage measured at the bushing tap adaptor with a reference voltage obtained from the same high voltage source, as described in U.S. Pat. No. 4,757,263. This method requires a separate connection to the high voltage source through for example an instrument transformer.
A second method commonly used for monitoring transformer bushings is the sum-of-current method. This method involves summing the currents from each bushing test tap for three phases; the sum is then relayed to a null meter, which detects differences in the sum-of-currents.
A third method currently used is illustrated in FIG. 2. This method involves relative measurements of a loss tangent and a relative amplitude between two bushings connected to the same phase but different transformers.
Hence, there exist a need for an improved method and system for monitoring transformer bushings that circumvent the above outlined problems and shortcomings.